1. Field of the Invention
The present invention relates to a lithium rechargeable battery, a composite particle for the lithium rechargeable battery, and a manufacturing method of the composite particle for the lithium rechargeable battery, more specifically to a technology for making composite particles of an active material having significant volume change.
2. Background Art
A lithium rechargeable battery is used as a main power source of a mobile communication device and a portable electronic device, because the battery has high energy density at a high voltage. In response to downsizing and performance improvement of the devices, the lithium rechargeable battery is required to also have higher performance, and many studies have been performed.
Many materials have been proposed as positive and negative active electrode materials of a lithium rechargeable battery. Various new materials have been studied as a negative electrode active material that allows further increase of the capacity. For example, in Japanese Patent Unexamined Publication No. H07-029602 and Japanese Patent Unexamined Publication No. 2001-291512, as a negative electrode material, single metal such as silicon (Si) and tin (Sn) capable of storing and emitting lithium ions, or alloys of these metals are proposed.
However, powder made of metals such as Si and Sn or an alloy thereof significantly expands and contracts in response to the storage and emission of lithium ions in charging and discharging reactions. Distortion due to the expansion and contraction degrades the conductive network in a mixture containing an active material, and reduces the battery characteristic. Therefore, Japanese Patent Unexamined Publication No. 2003-303588 discloses an example where a composite particle having a porous structure including clearances is formed of active material particles and the expansion of the composite particle by the active material particles is absorbed. In such a composite particle, however, only pores with a micro diameter exist homogeneously, and the active material particles are in tight contact with each other. Therefore, there is no spatial allowance for absorbing rapid expansion of the active material particle to moderate the expansion of the composite particle.
While, Japanese Patent Unexamined Publication No. H08-321300, for example, discloses an active material particle having a cavity formed in its center and a pore connecting to the cavity. The cavity formed in the particle can efficiently absorb distortion by expansion and contraction of the active material particle, and the high-rate charge and discharge characteristics seems to be improved by retaining electrolytic solution in the cavity.
This active material is made of carbonaceous material, and a so-called fine pore with a diameter of 1/10 or shorter of the active material particle diameter connects to the cavity. Therefore, when the active material particle is made of carbonaceous material that undergoes small expansion, such a particle structure presents no problem. When material that undergoes significant expansion is used as the active material, the expansion of the active material causes the fine pore to be blocked even when the cavity moderates the expansion and contraction of the active material. As a result, a part in contact with the electrolytic solution stored in the cavity separates from an ion conductive network. Therefore, a sufficient characteristic is not obtained especially in high-rate charge or discharge.